For many applications, Josephson devices are extremely useful. These applications include use as voltage standards, parametric amplifiers, and millimeter wave generation and detection circuits and provide motivation for putting Josephson devices in series. All of these applications work better if the Josephson devices are coherent, i.e., if the Josephson devices have their oscillations coupled to achieve phase locking. In the prior art, it is known that coherence can be obtained if the Josephson devices in a series chain of the devices interact with one another as shown in K. K. Likharev, Reviews of Modern Physics, 51, 1 (1979). Unfortunately, Josephson devices in series arrays and fed by a current source do not interact electrically in the absence of shunt elements.
The prior art contains references to different efforts to achieve coherence in series arrays of Josephson tunnel junctions. These references have described the use of either close proximity between neighboring junctions, such as that taught in Mercereau et al, Applied Physics Letters 25, 18, 467 (1974) and J. E. Mercereau et al, J. Appl. Phys. 44, 4 (1973), or the use of shunting elements, such as resistors or inductors, connected between neighboring pairs of junctions as described in J. E. Lukens et al, IEEE Transactions on Magnetics, MAG-15, page 462 (January 1979); J. E. Lukens et al, AIP Conference Proceedings No. 44, page 327 (Charlottesville, 1978); and J. E. Lukens et al, AIP Conference Proceedings No. 44, page 298 (Charlottesville, 1978).
In the Mercereau et al references, junctions are placed very closely together in distances of the order of microns in order to achieve cooperation and interaction between neighboring junctions. The mechanism for interaction between neighboring junctions is a direct interaction type of mechanism in which quasi-particles from one junction travel to the adjacent junction. This structure is characterized by a characteristic length which is the quasi-particle diffusion length. This diffusion length is approximately one micron and for this reason the junctions are in very close proximity to one another in order to ensure that quasi-particles from one junction will be able to diffuse to the neighboring junction. If one junction in the series is an electrical short, the series chain of junctions will be broken because quasi-particles from one junction may not be able to diffuse the longer distance past the shorted junction to the next adjacent unshorted junction. In this structure, coupling depends upon each Josephson junction being coupled to the next Josephson junction and coherence depends upon maintaining the coupling in the entire chain of junctions. In addition to the problem which arises when one of the junctions is an electrical short, it is difficult from a fabrication standpoint to provide junctions as close as required by the quasi-particle diffusion length, and this puts additional constraints on the processing techniques which can be used.
Josephson junctions arranged in series with one another and having DC current through them do not normally interact with one another. That is, each junction will operate independently of the other junctions and there will be no cooperative effect. In order to make them interact, and hopefully in a coherent manner, the close proximity approach of Mercereau et al is one of the techniques that have been suggested in the prior art. Another technique is the use of shunting elements as described in the aforementioned references to Lukens et al.
In the Lukens technique, the shunting elements give a coupling between pairs of Josephson junctions by creating circuit loops that obey Kirchoff's law that the sum of the voltages around the loop is equal to zero. In this reference, the coupling elements bridge adjacent pairs of Josephson junctions and are not located between single adjacent junctions. The technique of Lukens et al is limited in the frequencies of the waves which can be generated or detected by the series array, the upper limit being approximately 30 GHz. Also, the size of the array is very much smaller than the characteristic length of the structure, which is the wavelength of oscillation of the Josephson junctions. This array cannot provide controlled coherence above approximately 30 GHz and no resonance phenomena is involved in the coupling of one junction to another junction. In Lukens et al, nearest neighbor coupling only is provided by the shunting elements, and it is not possible to couple other than nearest neighboring junctions. The Lukens structure of series connected Josephson junctions and shunting elements is small compared to the wavelength of interest, and is therefore difficult to fabricate. Furthermore, its high frequency limit is significantly less than that which would be most desirable.
The present invention overcomes these prior art problems and provides series connections of Josephson devices in which coherence is obtained. The technique for providing coherence is one which uses indirect coupling between the Josephson devices of the array in that each of the devices interacts with the total structure, and more particularly with an electromagnetic wave(s) which occupies the entire structure. The Josephson devices are series elements in a transmission line (i.e., a waveguide type structure) and coupling of the Josephson devices is obtained by the electromagnetic waves present along the transmission line. This series array can operate coherently at frequencies above 100 GHz, and may potentially develop micro watts or more of power in the submillimeter regime.
In contrast with the prior art techniques, the present technique does not use the proximity effect of closely spaced junctions and is therefore not limited to separations of Josephson devices within the quasi-particle diffusion length. Furthermore, the present technique is not limited in frequency and does not depend upon nearest neighbor coupling to provide coherence. The size of the entire array can be much greater than the characteristic length of interest, which in this case is the wavelength of the electromagnetic wave sustained along the line. Thus, even if some of the Josephson devices are electrical shorts, coherence will still be obtained in the array.
Accordingly, it is a primary object of the present invention to provide an improved technique and apparatus for achieving coherence in an array of devices which exhibit Josephson currents therethrough.
It is another object of the present invention to provide a coherent array of Josephson devices which will deliver substantial power at frequencies at least 100 GHz.
It is another object of the present invention to use electromagnetic waves to couple an array of Josephson devices in a manner to obtain coherence.
It is a further object of the present invention to provide a series array of Josephson devices exhibiting coherence, where the total length of the array is greater than the characteristic length of the array.
It is a still further object of the present invention to provide a series array of Josephson devices which exhibits coherence even if some of the Josephson devices do not operate properly.
It is another object of the present invention to provide a series array of Josephson devices which operates coherently without the need for nearest Josephson device interaction.